An antenna apparatus is provided in various communication devices to transmit and receive radio waves of a predetermined frequency band. The popularity and the range of application of portable radio devices represented by mobile telephones have grown, and therefore there is a growing demand for wideband capability of portable radio device antennas. Also, with the rapid spread of portable radio devices, the number of channels in one wireless communication system is likely to be insufficient. Consequently, studies are underway to use one system together with other wireless communication systems using different frequency bands to secure a necessary number of channels. With significant advances in techniques for reducing size and weight, as a terminal, a single portable radio device that can use two kinds of wireless communication systems have been developed. Moreover, a bandwidth of several hundreds MHz is considered to be necessary in the UHF band for, for example, reception of terrestrial digital broadcast. Furthermore, to reduce the size of a radio apparatus by supporting a plurality of WLAN (Wireless Local Area Network) standards using different frequency bands by means of a single antenna, for example, an antenna covering the 2.4 GHz and 5.2 GHz bands is required.
In communication between a mobile station and base station in a mobile communication system, fading often occurs in which the received signal level fluctuates depending on various radio wave propagation environments. Effective measures to take against fading include providing antenna selective diversity and combined diversity. With antenna selective diversity, a plurality of antennas are generally installed, and communication is performed by selecting an antenna with good conditions when the receiving state degrades due to fading. For example, there are antenna selective diversity apparatuses with a TDMA (Time Division Multiple Access) communication scheme in which the same frequency is shared by a plurality of users by means of time division.
Also, in recent wireless communication systems, MIMO communication has attracted attention as a technique to improve the efficiency of transmission. In MIMO communication, a transmitting apparatus having a plurality of transmitting antennas simultaneously transmit streams comprised of a plurality of bits from these transmitting antennas, and a receiving apparatus having a plurality of receiving antennas demultiplex and demodulate those streams from the transmitting apparatus. Therefore, when mutually different streams are simultaneously transmitted from all of the transmitting antennas of the transmitting apparatus, compared to the case of one transmitting antenna, theoretically, it is possible to increase the efficiency of transmission to the multiples of the number of transmitting antennas.
In communication providing a plurality of antenna elements and adopting diversity operations, MIMO communication or adaptive array antenna operations, the directivity of a single antenna alone is not sufficient to cope with the fluctuation of polarization conditions which change every minute between a base station and a terminal. In particular, the directivity of a single antenna alone is not sufficient to cope with the fluctuation of polarization characteristics of received signals. To secure the communication capacity, an antenna system is necessary which flexibly responds to the fluctuation of polarization characteristics.
As measures to take against the above, Patent Documents 1 to 4 disclose techniques to improve communication capacity utilizing polarization.
Patent Document 1 discloses a communication apparatus and method using adaptive array antennas by which a base station switches polarization characteristics and in which the terminal side performs reception with a polarization matching a polarization characteristic from the base station. With the apparatus disclosed in Patent Document 1, if the base station outputs a vertically polarized wave, the terminal receives this with vertical polarization. By this means, antenna polarization matches between the terminal and the base station, so that it is possible to improve communication capacity. Further, by performing communication using polarizations which change between terminals, it is possible to suppress the interference for communication paths between the terminals and the base station.
Patent Document 2 discloses a radio wave transmitting and receiving apparatus for improving communication capacity by providing a plurality of antenna elements of different polarizations in a base station, implementing selective diversity and performing MIMO operations using this antenna group.
Patent Document 3 discloses an antenna apparatus that performs directivity synthesis using a plurality of antenna elements from a large number of antenna elements and then performs MIMO communication.
Patent Document 4 discloses a MIMO wireless communication system for providing a plurality of antenna elements of different polarizations both in a base station and in a terminal, and performing MIMO. By this means, the directivity and polarization change between antenna elements for transmission and reception in MIMO propagation paths, and the separation level between propagation paths improves, so that it is possible to improve communication capacity.    Patent Document 1: Japanese Patent Application Laid-Open Number 2004-80353    Patent Document 2: Japanese Patent Application Laid-Open Number 2004-312381    Patent Document 3: Japanese Patent Application Laid-Open Number 2005-86518    Patent Document 4: Japanese Patent Application Laid-Open Number 2004-517549